J.J. Kneen's Placenames of the Isle of Man has parish maps of all the Manx parishes, but these can be difficult to relate to the features on the ground. In fact, the Ordnance Survey maps have the parish boundaries marked - but these are not on the current edition, sadly. The 1963 (seventh series) and 1970 maps - both sheet 87 - have them if you can find a copy. In my experience (from elsewhere and for other reasons) they are detailed enough to fit the boundaries to 1:25 000 maps showing the field bounderies if needed, in most cases.